Alkenes: a carbon–carbon double bond makes them unsaturated
Definition
Carbon-carbon double bond
Two covalent bonds that join the same two carbon atoms.
- Alkenes are hydrocarbons that contain a carbon–carbon double bond, written C=C\text{C}=\text{C}C=C.
- Alkenes are unsaturated, because each molecule has two fewer hydrogen atoms than the alkane with the same number of carbons.
- Every carbon atom still forms four bonds in total.
- The double bond uses one extra bond between the two carbons, leaving room for two fewer carbon–hydrogen bonds.
Key Idea
- The carbon–carbon double bond is the feature that identifies an alkene.
- For the same number of carbons, an alkene has two fewer hydrogen atoms than the matching alkane.
Every alkene fits the general formula CnH2n\text{C}_n\text{H}_{2n}CnH2n
Definition
General formula
Letters and numbers that show the pattern followed by the molecular formulae of a homologous series.
- Alkenes form a homologous series, so they share one general formula.
- The general formula of the alkenes is CnH2n\text{C}_n\text{H}_{2n}CnH2n.
- The letter nnn is the number of carbon atoms in one molecule.
- The number of hydrogen atoms, 2n2n2n, is always twice the number of carbons.
- The smallest alkene has n=2n=2n=2, because a C=C\text{C}=\text{C}C=C bond needs at least two carbon atoms.
Example
- If n=4n=4n=4, the alkene has four carbon atoms.
- The number of hydrogens is 2×4=82\times4=82×4=8, giving C4H8\text{C}_4\text{H}_8C4H8.
- The alkane with four carbons is C4H10\text{C}_4\text{H}_{10}C4H10, which has two more hydrogens.
The first four alkenes are named from their carbon count
- Ethene has two carbons, C2H4\text{C}_2\text{H}_4C2H4.
- Propene has three carbons, C3H6\text{C}_3\text{H}_6C3H6.
- Butene has four carbons, C4H8\text{C}_4\text{H}_8C4H8.
- Pentene has five carbons, C5H10\text{C}_5\text{H}_{10}C5H10.
- You need these four names only, not other individual alkenes.
- There is no “methene”, because one carbon atom cannot form a carbon–carbon double bond.
Note
- Alkene names end in -ene, while alkane names end in -ane.
- Match the carbon count to the name: two gives ethene, three propene, four butene, five pentene.
Molecular formulae count atoms; displayed formulae show the double bond
Definition
Displayed formula
A diagram that shows all the atoms in a molecule and all the covalent bonds between them.
- A molecular formula gives the number of each atom in one molecule, such as C3H6\text{C}_3\text{H}_6C3H6 for propene.
- A molecular formula does not show where the double bond is.
- A displayed formula shows every atom and every bond, including the C=C\text{C}=\text{C}C=C.
- The displayed formula of propene is shown here.
- HHH∣∣∣H−C=C−C−H∣H\begin{array}{ccccccc} \text{H} & & \text{H} & & \text{H} & & \\ | & & | & & | & & \\ \text{H}-\text{C} & = & \text{C} & - & \text{C} & - & \text{H} \\ & & & & | & & \\ & & & & \text{H} & & \end{array}H∣H−C=H∣C−H∣C∣H−H
- Counting the atoms gives three carbons and six hydrogens, so the molecular formula is C3H6\text{C}_3\text{H}_6C3H6.
Exam technique
- Do not call C3H6\text{C}_3\text{H}_6C3H6 a displayed formula; a displayed formula must show every bond.
- When you draw an alkene, put in the C=C\text{C}=\text{C}C=C and check every carbon has exactly four bonds.
Recognising an alkene: check CnH2n\text{C}_n\text{H}_{2n}CnH2n and look for C=C\text{C}=\text{C}C=C
- From a molecular formula, check whether the hydrogens equal twice the carbons.
- For example, C5H10\text{C}_5\text{H}_{10}C5H10 fits CnH2n\text{C}_n\text{H}_{2n}CnH2n because 2×5=102\times5=102×5=10.
- From a displayed formula, check the molecule has only carbon and hydrogen and contains a C=C\text{C}=\text{C}C=C.
- Match two, three, four or five carbons to ethene, propene, butene or pentene.
Self review
- What structural feature do all alkenes contain?
- What is the general formula of the alkenes?
- Why are alkenes described as unsaturated?
- Give the names and molecular formulae of the first four alkenes.
- What does a displayed formula show that a molecular formula does not?
